Method and apparatus for the detection of selected components in fluids

ABSTRACT

A component of a fluid mixture, for example mercury vapor in air, is detected by selectively adsorbing the component onto a conductive thin layer of molecular thickness of a material having a chemical affinity for such component, and observing the resultant change of electrical resistivity of the layer.

[ 1] 3,714,562 Jan. 30, 1973 [54] METHOD AND APPARATUS FOR THE DETECTIONOF SELECTED COMPONENTS IN FLUIDS [75] Inventor: John J. McNerney, Tempe,Ariz.

[73] Assignee: Selco Mining Corporation Limited,

Toronta, Ontario, Canada [22] Filed: April 23, 1971 [21] Appl. No.:137,684

[52] U.S. Cl. ..324/65 R, 23/254 E, 73/27, 338/34 [51] Int. Cl. ..G0ln27/04, HOlc l3/O0 [58] Field of Search ..324/65, 71; 23/254 E; 73/27;338/34 [56] References Cited UNITED STATES PATENTS 3,625,756 12/1971Taguchi ..324/65 R 3,558,279 l/l97l OTHER PUBLICATIONS I Wilhelmsen etal.; Jour. Vacuum Sci & Tech; Vol. 7 No. 1;Jan-Feb. i970; pp. 39-42.

McRae et al ..23/254 E Primary EtaminerAlfrd E. Smith Attorney-Maybee &Legris [57] ABSTRACT A component of a fluid mixture, for example mercuryvapor in air, is detected by selectively adsorbing the component onto aconductive thin layer of molecular thickness of a material having achemical affinity for such component, and observing the resultant changeof electrical resistivity of the layer.

16 Claims, 7 Drawing Figures PATENTEDJMOIBB 3 714 562 SHEET 20F 2 I mlnvenfor J hn I. McNerney Ahorney METHOD AND APPARATUS FOR THE DETECTIONOF SELECTED COMPONENTS IN FLUIDS BACKGROUND OF THE INVENTION Thisinvention relates to the detection of selected components in fluidmixtures and is applicable, for example, to the detection andmeasurement of minute traces of mercury vapor, iodine vapor, or othersubstances in atmospheric air or in soil gas.

The detection of minute traces of selected components in gaseousmixtures, for example mercury vapor in air, has hitherto relied uponspectrophotometric techniques which, although extremely sensitive, havecertain serious limitations. The most serious limitation, especially inthe case of mercury vapor detection, is usually set by the presence ofinterfering substances which absorb electromagnetic energy in thespectral region containing the mercury line. Another serious limitation,where such detection is to be carried out in the field, is the need foran extremely stable voltage source which in effect limits the trueportability of any spectrophotometric instrument operating at theextremely high levels of sensitivity required for air and soil gassampling.

The technique of the present invention represents a radically newapproach to the problem of detecting minute traces of vapor in soil gasor in air, and may be applied readily in the field and in thelaboratory.

SUMMARY OF THE INVENTION The present invention is based on the discoverythat when a fluid is adsorbed onto a thin layer of conductive materialhaving a chemical affinity for the fluid, the electrical resistance ofthe layer rises appreciably. For example, a thin film of gold having athickness less than the mean free path of electrons undergoes asignificant rise in resistivity when mercury vapor is adsorbed onto thefilm, the adsorbed mercury atoms isolating electrons that were formallyavailable for electronic conduction. In the case of a gold film, thefilm thickness is preferably between 75 and 300 A.U., but may be as highas 700 1,000 A.U. The thicker the layer, the less noticeable is theresistance change since the film properties of the layer become maskedat greater thicknesses by the properties of the bulk material.

The layer of conductive material is herein described as a thin layer ofmolecular thickness, by which is meant that the observable physicalcharacteristics of the layer are determined primarily by the twodimensional configuration of the material and are not the properties ofthe material in bulk.

According to the present invention, the presence ofa selected componentin a fluid mixture is detected by selectively adsorbing the componentfrom the fluid mixture onto a thin layer of molecular thickness of anelectrically conductive material having a chemical affinity for thecomponent, and detecting the change of resistance of the layerconsequent to the adsorption.

In the case of a gold film for the adsorption of mercury vapor, it hasbeen found that the film resistance increases as soon as the film isexposed to the vapor, the resistance change being a function of thevapor concentration. In the course of time, however, the film resistancecontinues to increase very slowly; it is believed that this furthereffect, which is quite distinct from the phenomenon on which theinvention is based, is due to amalgamation with the gold.

It is to be understood that metals other than gold may be used in themethod of the invention. Thus mercury vapor may be detected using,instead of a gold layer, a

thin layer of molecular thickness of any metal which has a chemicalaffinity for mercury. For the detection of mercury vapor in air,however, it is desirable that the metal should be one of the noblemetals. Furthermore, other vapors than mercury can be detected by meansof this method; the thin layer of conductive material must have achemical affinity for the vapor to be detected, of course, and incertain circumstances it may be necessary to provide a filter for theremoval of substances other than the particular vapor to be detected, ifsuch other substances might be present and would otherwise be adsorbedonto the layer. Thus iodine vapor may be detected using, instead of agold layer, a thin layer of silver of molecular thickness; if mercuryvapor might be present in the gas under examination in such a case, itwould be necessary to provide a suitable filter for the selectiveremoval of mercury vapor prior to contacting the gas with the layer.

BRIEF DESCRIPTION OF THE DRAWINGS One embodiment of the invention willnow be described by way of example with reference to the accompanyingdrawings, in which:

FIG. 1 illustrates an apparatus used to detect mercury vapor in soilgas, together with a pump (shown in section) for extracting apredetermined volume of gas from the soil;

FIG. 2 is a side elevation, partly broken away, of the apparatus shownin FIG. 1;

FIG. 3 is a part sectional plan view of a detail of the apparatus;

FIG. 4 is a section on line 44 in FIG. 3;

FIG. 5 is a section on line 55 in FIG. 3;

FIG. 6 illustrates a component element of the apparatus; and

FIG. 7 is a wiring diagram of a circuit used to detect or measureresistance changes due to the adsorption of mercury vapor.

The assembly illustrated in FIG. 1 comprises a pump 10 for extractingsamples of soil gas from the ground 11, and delivering the gas samplesto a detecting apparatus 12 by way of a tubular connection 13.

In practice it is found desirable to provide an intermediate stagebetween the pump and the apparatus 12. The intermediate stage mayconsist of a casing containing a silver mesh to collect mercury byadsorption from the gas sample, and a heater to drive off the adsorbedmercury vapor. The stage is first connected to the pump to receive thesample, and is subsequently connected to the apparatus, the adsorbedmercury vapor being driven off and passed into the apparatus. The pump10 comprises a cylinder portion 14 in which a hand-operated plunger 15can slide, a tubular probe portion 16 which is adapted to be driven intothe soil to a depth determined by a pair of lateral arms 17, and anintermediate portion 18. Although the portions l4, l6 and 18 are shownin the drawing as being in one piece, they are preferably separatecomponents that can be readily coupled together. To obtain a gas sample,the probe portion 16 is driven into the soil to the appropriate depthand the plunger is raised from the bottom to the top of the cylinder 14.A predetermined volume of soil gas thereby passes into the cylinder 14by way of a spring-loaded check valve 19, a vacuum gauge being used toindicate the pressure within the tubular probe and so provide anindication of the required quantity of soil gas having been extracted.By pushing down the plunger 15, the soil gas is subsequently expelledfrom the cylinder portion 14 via a second spring-loaded check valve 21and through the tubular connection 13 and to the apparatus 12 preferablyby way of the intermediate stage referred to.

The apparatus 12 provides a box-like casing which houses the componentsof the apparatus as described hereinafter, and an instrument panel 22which is protected by a lid 23, the lid 23 having a handle 24 for thepurpose of carrying the apparatus. On the instrument panel 22 are showna volt meter 24, an ON-OFF switch 26, a sensitivity selector control 27,zero adjustment controls 28, a drift compensator switch 29 and control30, battery test terminals 31, and terminals 32 for connection to anexternal recorder when required.

Located in the bottom part of the casing of the apparatus 12 is abattery of mercury cells providing a stable source of voltage for thebridge circuit shown in FIG. 7. A first metal block 33 mounted withinthe casing of the apparatus 12 has a flat upper face which is recessedto provide respective seatings for a pair of sensor elements 34, 35 andshallow spaces 36, 37 immediately above the detector elements. A secondmetal block 38 is clamped to the upper face of the first block 33 byscrews 39, thus covering the elements 34, 35 and closing the spaces 36,37. The block 38 is formed with a pair of identical tubular passages 40,41 which communicate with the spaces 36 and 37 respectively by series ofvery fine orifices 42, 43. One end of the passage is connected to thetubular connection 13 by way of an adaptor 44, and gas flowing into thepassage 40 by way of the tubular connection and adaptor flows throughthe orifices 42 onto the upper surface of the sensor 34. The space 36 isdivided longitudinally by a baffle 45 depending from the bottom face ofthe block 33, so that all gas passing through the space 36 is forced toflow across the upper surface of the sensor 34, and in close proximityto it, before passing to an outlet 46. The outlet 46 is connected by wayofa connecting passage containing a mercury vapor filter 47 to an inlet48 at one end of the passage 41. The mercury vapor filter may be of thesilver mesh type. Gas flowing into the passage 41 flows through theorifices 43 into the space 37 and is compelled to flow very close to thesurface of the sensor element 35 by reason of a second barrier disposedin the same manner as the barrier 45. The gas which passes from thespace 37 is discharged to atmosphere through an outlet 50.

Each of the sensor elements 34, 35 comprises a thin rectangular glassplate 51 (FIG. 6) constituting a substrate on which a thin layer of goldmolecular thickness has been deposited, the configuration of the goldlayer 52 being such as to provide a reasonably large surface area ofgold and a reasonably long resistance path between terminals 53, 54. Inoperation of the apparatus the resistance of the gold layer betweenterminal electrodes 53 and 54 is caused to change by reason of theadsorption of mercury vapor and other vapors onto it,

the resistance change being a function of the concentration of thevapors adsorbed. This resistance change in the first sensor element 34is to be compared with the resistance change of the sensor element 35when the same gas sample flows over the gold layer thereon. Sincemercury vapor is prevented from reaching the element 35 by the filter47, any difierence between the resistance changes must be due to theadsorption of mercury vapor on the surface of the first detectingelement alone.

In the apparatus of the present example the gold layer forming theresistance path of each sensor element should preferably be from 75 A.U.to 300 A.U. in thickness, giving a resistance of 500 to 70 ohms.

FIG. 7 is an electrical circuit diagram of the apparatus. Theresistances formed by the sensors 34 and 35 are indicated at 340, 35a.These resistances are connected in two arms of a resistance bridgecircuit having a pair of ratio arms formed by a resistance chain 61 anda set of resistances 62. Selection of the resistance values of the ratioarms is made by means of a pair of gauged selector switches 27a, 27boperated by the sensitivity selector control 27 (FIG. 1). The bridgecircuit is supplied from a battery of mercury cells 63, to which it isconnected by respective contacts 26a, 26b, 26c and 26d of the ON-OFFswitch 26 (FIG. 1). Out of balance bridge current is converted to avoltage signal and amplified by first and second operational amplifiers64, 65, and applied to the center-zero voltmeter 25. Coarse, medium andfine controls for zero adjustment of the apparatus are furnished byrespective potentiometers 28a, 28b and 280 connected between the sensorresistances 34a, 35a and adjusted by means of the zero adjustmentcontrols 28 (FIG. 1 To provide compensation for d.c. drift anoperational amplifier 66 is connected in a drift compensation circuit 67including a variable resistance 30a, which is set by the compensationcontrol 30 of FIG. 1, and switch contacts 29a, 29b, which are operatedby the compensation switch 29 of FIG. 1. Also represented in FIG. 7 area pair of switches 31a, 31b operated by the pushbuttons 31 for batterytesting, and the pair of terminals 32 to connecting an external recorderto the apparatus.

In order to use the apparatus for the detection of mercury vapor in soilgas, the pump is connected to the intermediate stage previously referredto, and the pump probe is driven in the ground to the required depth. Apredetermined quantity of soil gas is extracted by means of the pump andpassed via the tubular connection 13 to the intermediate stage, whereany mercury vapor is adsorbed onto the silver mesh collector of thestage. After setting up the apparatus by means of the zero adjustmentand drift compensation controls, the adsorbed vapor is driven off byheating, and then is carried by a gas current and passed via the tubularconnection 13 to the gas flow path within the apparatus, the gas flowingsuccessively over the gold layer of the sensor element 34, through thesilver mesh filter 47 where any residual mercury vapor is removed, andthen over the gold layer of the reference sensor element 35. In generalthe resistance values of the sensor elements 34 and 35 may change due tothe adsorption of vapors from the gas sample onto the gold layers. Thesensor elements are practically identical in form and configurationandhave substantially identical adsorption-resistance characteristics.However, if mercury vapor is present in the gas sample, this is adsorbedonto the gold layer of the first sensor element 34 only, as it isprevented from reaching the second sensor element 35 by the filter 47.The detection of mercury vapor therefore produces a differentialresistance change, the magnitude of which is an indication of theconcentration of mercury vapor. As a result of the differentialresistance change the bridge circuit is unbalanced, the amount ofunbalance being indicated by the voltmeter 25, which can therefore becalibrated in terms of mercury vapor concentration.

Adsorbed mercury and other adsorbed substances can subsequently beremoved from the gold layers by heating.

The apparatus may be readily adapted for the detection of vapors otherthan mercury, either in soil gas or in atmospheric air. The conductivethin layers of the sensor elements must of course be of a material whichhas a chemical affinity for the particular vapor or vapors to bedetected, and the filter 47 must be capable of selectively removing suchvapor or vapors. For example, for the detection of iodine vapor in airor soil gas, the conductive thin layers should preferably be of silver,and the filter 47 may take the form of a porous silver body providing alarge surface area in the gas flow path for the adsorption of residualiodine vapor. Since silver has also a chemical affinity for mercury, itmay be necessary to place a suitable mesh filter at the inlet to thepassage 40 (FIGS. 3 and 5) so as to remove any mercury vapor which mightbe present in the gas under examination.

Although the invention has been particularly described with reference tothe detection of mercury vapor or iodine vapor in gaseous mixtures, itis not limited thereto in its applications. Broadly speaking theinvention is readily applicable to the detection of a selected componentin a gas or liquid by passing the gas orv liquid over a thin layer ofmolecular thickness of a material onto which the component is adsorbed,and observing the resultant change in electrical resistance of thelayer. For convenience of observing and interpreting the resultantresistance change, the detection apparatus may include a pair ofidentical sensor elements which are connected electrically in aresistance bridge circuit, and physically mounted so that the fluidunder examination passes over the sensor elements in succession, anyresidual component to be detected being filtered from the fluid beforethe fluid passes over the second sensor element, which provides areference.

What I claim as my invention is:

l. A method of detecting the presence of a selected component in a fluidmixture which comprises selectively adsorbing the component from themixture onto a thin layer of molecular thickness of a metal having achemical affinity for the component, and detecting the change ofresistance of the layer consequent to the adsorption.

2. A method of detecting the presence of a selected component in a gas,which comprises passing an electric current through a thin layer ofmolecular thickness of a metal having a chemical affinity for thecomponent to be detected, passing a predetermined quantity of the gasover the layer so as to remove such component therefrom by chemisorptiononto the layer, and measuring the change in the electric current passingthrough the layer.

3. The method claimed in claim 2, wherein the component to be detectedis mercury vapor and the metal is one having an affinity for mercury anda capacity for forming solid solutions therewith.

4. The method claimed in claim 3, wherein the metal is a noble metal.

5. The method claimed in claim 4, wherein the metal is gold.

6. The method claimed in claim 2, wherein the component to be detectedis iodine vapor and the metal is silver.

7. Apparatus for detecting the presence of a selected component in agas, comprising: means defining a substrate, a thin layer of molecularthickness of a metal having a chemical affinity for the component, thelayer being deposited on the substrate, means for passing the gas overthe layer so that such component is removed from the gas by selectiveadsorption onto the layer, and circuit means connected to the layer fordetecting the change of resistance of the layer consequent to theadsorption.

8. Apparatus for detecting the presence of a selected component in agas, comprising:

means defining a substrate;

a thin layer of molecular thickness of a metal having a chemicalaffinity for the component deposited on the substrate; a pair ofelectrodes making contact with the layer; circuit means connectedbetween the electrodes for passing electric current through the layer;and

means defining a gas flow path for causing the gas to flow over thelayer whereby the component to be detected is removed from the gas bychemisorption onto the layer,

said circuit means including current responsive means for indicating achange in current consequent to a change in the resistance of the layer.

9. Apparatus according to claim 8 for the detection of mercury vapor inair, wherein the metal is one having an affinity for mercury and acapacity for forming solid solutions therewith.

10. Apparatus according to claim 9, wherein the metal is a noble metal.

11. Apparatus according to claim 10, wherein the metal is gold.

12. Apparatus according to claim 8, for the detection of iodine vapor inair, wherein the metal is silver.

13. Apparatus for detecting the presence of a selected component in agas comprising:

first and second identical sensor means:

each of said sensor means consisting of a substrate supporting a thinlayer of molecular thickness of a metal having a chemical affinity forthe component to be detected, and a pair of electrodes making contactwith the layer;

circuit means connected with the pairs of electrodes and adapted forcomparing resistance changes of the respective sensor means;

structural means defining a gas flow path for the passage of said gas,the gas flow path consisting of fist and second series-connectedpassages and a connecting passage therebetween 14. Apparatus accordingto claim 13 for the detection of mercury vapor in air, wherein the metalof the layers is a noble metal.

15. Apparatus according to claim 14, wherein the metal of the layers isgold.

16. Apparatus according to claim 13 for the detection of iodine vapor inair, wherein the metal of the layers is silver.

1. A method of detecting the presence of a selected component in a fluidmixture which comprises selectively adsorbing the component from themixture onto a thin layer of molecular thickness of a metal having achemical affinity for the component, and detecting the change ofresistance of the layer consequent to the adsorption.
 2. A method ofdetecting the presence of a selected component in a gas, which comprisespassing an electric current through a thin layer of molecular thicknessof a metal having a chemical affinity for the component to be detected,passing a predetermined quantity of the gas over the layer so as toremove such component therefrom by chemisorption onto the layer, andmeasuring the change in the electric current passing through the layer.3. The method claimed in claim 2, wherein the component to be detectedis mercury vapor and the metal is one having an affinity for mercury anda capacity for forming solid solutions therewith.
 4. The method claimedin claim 3, wherein the metal is a noble metal.
 5. The method claimed inclaim 4, wherein the metal is gold.
 6. The method claimed in claim 2,wherein the component to be detected is iodine vapor and the metal issilver.
 7. Apparatus for detecting the presence of a selected componentin a gas, comprising: means defining a substrate, a thin layer ofmolecular thickness of a metal having a chemical affinity for thecomponent, the layer being deposited on the substrate, means for passingthe gas over the layer so that such component is removed from the gas byselective adsorption onto the layer, and circuit means connected to thelayer for detecting the change of resistance of the layer consequent tothe adsorption.
 8. Apparatus for detecting the presence of a selectedcomponent in a gas, comprising: means defining a substrate; a thin layerof molecular thickness of a metal having a chemical affinity for thecomponent deposited on the substrate; a pair of electrodes makingcontact with the layer; circuit means connected between the electrodesfor passing electric current through the layer; and means defining a gasflow path for causing the gas to flow over the layer whereby thecomponent to be detected is removed from the gas by chemisorption ontothe layer, said circuit means including current responsive means forindicating a change in current consequent to a change in the resistanceof the layer.
 9. Apparatus according to claim 8 for the detection ofmercury vapor in air, wherein the metal is one having an affinity formercury and a capacity for forming solid solutions therewith. 10.Apparatus according to claim 9, wherein the metal is a noble metal. 11.Apparatus according to claim 10, wherein the metal is gold. 12.Apparatus according to claim 8, for the detection of iodine vapor inair, wherein the metal is silver.
 13. Apparatus for detecting thepresence of a selected component in a gas comprising: first and secondidentical sensor means: each of said sensor means consisting of asubstrate supporting a thin layer of molecular thickness of a metalhaving a chemical affinity for the component to be detected, and a pairof electrodes making contact with the layer; circuit means connectedwith the pairs of electrodes and adapted for comparing resistancechanges of the respective sensor means; structural means defining a gasflow path for the passage of said gas, the gas flow path consisting offist and second series-connected passages and a connecting passagetherebetween the first sensor means being mounted in the first passage,the second sensor means being mounted in the second passage, and theconnecting passage containing a filter for the removal of such componentresidual in the gas flowing from the first passage to the secondpassage, whereby the said component is selectively adsorbed onto thelayer of the first sensor means.
 14. Apparatus according to claim 13 forthe detection of mercury vapor in air, wherein the metal of the layersis a noble metal.
 15. Apparatus according to claim 14, wherein the metalof the layers is gold.